Would the pattern difference between a 28" barrel with a modified choke. And a 28" barrel with a cylinder choke be that much noticeable put at let's say..... 15 to 25 yards. If so, how much would it be?

Yes, there would be observable differences in the patterns at those distances. Maybe not so much at 15 yards, but definitely would see a large change at 25 yards. How much the patterns expand depends on the type of ammunition and how it performs in your shotgun.